It has been known to the art that glycidyl acrylate or glycidyl methacrylate (hereinafter referred to collectively as "glycidyl (meth)acrylate") is synthesized by causing an alkali metal salt of acrylic acid or methacrylic acid (hereinafter referred to collectively as "(meth)acrylic acid") and epichlorohydrin of an amount in the range of 3 to 10 mols per mol of the aforementioned alkali metal salt to react with each other in the presence of about 0.3 to 2.0 mol% of a quaternary ammonium salt as a catalyst and about 0.01 to 0.2 wt% of a polymerization inhibitor, both based on the amount of the aforementioned alkali metal salt of (meth)acrylic acid, at a temperature in the range of 80.degree. to 120.degree. C. for a period in the range of 1 to 5 hours. The crude glycidyl (meth)acrylate obtained consequently is filtered or washed with water to remove therefrom an alkali metal chloride occurring as a by-product of the reaction, then distilled to expel therefrom any excess of epichlorohydrin, and finally purified by vacuum distillation to become a finished product.
In the glycidyl (meth)acrylate thus obtained as a finished product, however, epichlorohydrin still persists in a concentration approximately in the range of 0.1 to 1.0% by weight, with the inevitable result that the purity of glycidyl (meth)acrylate is lowered and the content of free chlorine is increased. Such is the true state of affairs. Since glycidyl (meth)acrylate is mainly used as a raw material for coating materials and results, the aforementioned increase of the free chlorine content results in acceleration of the corrosion of coated metallic substrates and loss of the economic worth of glycidyl (meth)acrylate. Further, epichlorohydrin possesses toxicity manifested in the form of violent stimulation of the skin. The glycidyl (meth)acrylate which contains this toxic compound demands particularly meticulous care to be exercised during the handling thereof. Thus, it has room for further improvement from the standpoints of safety, hygiene, and ecology.
Heretofore, as regards methods for the purification of glycidyl (meth)acrylate, various proposals have been made which are mainly aimed at heightening the yield of purification. For example:
(1) A method which converts the reaction mixture into a finished product by adding water to the reaction mixture, hydrolyzing the resultant mixture, and then distilling the organic phase separated in consequence of the hydrolysis (as disclosed in Japanese Patent Publication No. 28762/1970.
(2) A method which promotes stabilization of glycidyl (meth)acrylate resulting from the reaction by the addition thereto of either a sulfonate, an alkyl sulfuric ester or a sulfonate-form cation-exchange resin (as disclosed in Japanese Patent Application Laid-open No. 72115/1973), and
(3) A method which promotes stabilization of glycidyl (meth)acrylate resulting from the reaction by the addition thereto of either an alkali metal salt of mono- or di-nitrophenol or an alkali metal salt of a mono-, di-, or tri-nitrobenzoic acid (as disclosed in Japanese Patent Publication No. 42075/1982) have been introduced to the art.
These prior publications made absolutely no mention of a process for the production of glycidyl (meth)acrylate containing substantially no epichlorohydrin. The inventions disclosed thereby have objects entirely different from those of the present invention.
As means of producing glycidyl (meth)acrylate without use of epichlorohydrin, methods resorting to transesterification of methyl acrylate or methyl methacrylate with glycidol have been proposed (as disclosed in Japanese Patent Application Laid-open No. 15420/1975, Japanese Patent Publication No. 6133/1978, Japanese Patent Publication No. 38421/1972, French Pat. No. 2,088,971, Japanese Patent Application Laid-open No. 25174/1977, Japanese Patent Application Laid-open No. 3007/1979, and Japanese Patent Application Laid-open No. 118075/1981).
Since glycidol as the starting material possesses poor shelf life and undergoes degradation of purity with elapse of time even at room temperature, these methods are not suitable for stable production of glycidyl (meth)acrylate on a commercial scale.
As another conceivable method, precision distillation of the ordinary product may attain in some measure the same primary object as contemplated by the present invention. This method, however, is unfit for mass production of the compound on a commercial scale because the application of heat required for the distillation not merely lowers the yield heavily but also entails a serious economic loss.